In the US soil classification, major criteria of an argillic horizon are particle size distribution relative to an overlying horizon and either clay skins on ped surfaces or oriented clay occupying 1% or more of the cross section. On many occasions Indian pedologists while working in the micaceous IGP soils of the north-western IGP have however, experienced clay-enriched textural B-horizons but found no identifiable clay skins by a 10X hand lens. According to the US Soil Taxonomy, the clay illuviation process results in optically oriented pure void argillans. However, in IGP soils translocation of the fine clay particles results in textural pedofeatures of the ‘impure’ clay pedofeatures’ as a typical pedofeatures. Criteria of clay skins and pure clay pedofeatures (i.e. void argillans) for argillic horizon of the US Soil Taxonomy become incompatible in most of the Indian SAT soils. Indian pedologists have proved that while the presence of clay skins and pure clay pedofeatures is a clear evidence of illuviation, the absence of clay skins and presence of impure clay pedofeatures, does not necessarily mean that there has been no clay illuviation in the uniform parent material on a stable and more than 2500 year old geomorphic surface as the major pedogenic process under SAT climates. In view of the fulfillment of the textural requirement of a Bt horizon for such loamy IGP soils, would it be prudent to waive the clay skins criterion for applying the precise and unambiguous definitions of soil taxa? I invite my fellow pedologists for their valuable comments/suggestions.
I had the same problem frequently in my field works. As I was characterising soil mainly for agronomic purposes, so I had no possibility to make micromorphology studies, however i had situations, when I could observe something similar to "clay skins" or "clay films" (the Brazilians call it frequently "cerosidade", although they define it more broadly) not only in Bt horizons. Sometimes such things may be observed with magnifier, or sometimes even without this (see for example respective photos in American "Illustrated Guide to Soil Taxonomy", Brazilian "Manual tecnico de pedologia", both available as pdf in internet, or also Brazilian "Manual de descricao e coleta do solo no campo"). Certainly, without micromorphological laboratory I could not confirm my observations.
Personally I cannot agree, that the presence of so called "clay skins" should be the condition of classifying soil as Luvisol or other soil with argillic horizon, because:
1. clay skins may be formed after one intensive rain (see definition of horizon "Bt textural" in Brazilian soil classification);
2. is there any evidence, that clay skins are not present in C horizons? Any studies concerning this subject? May be they exist, but I would like to know them, so I will be very obliged for them. I am also pretty sure, that in eroded and intensivrly cultivated soils, when former Bt is horizon is being mixed with constantly renewed A horizon (due to tillage), the "clay films" will be detectable even in A horizon (actually - mixture of Ap and Bt, which should be designated A/B, or, more strictly Ap/Bt);
3. in majority of classifications (Soil Taxonomy, WRB, Brazilian) it is clearly stated, that Bt horizon may be formed not only by clay iluviation, but also, but clay destruction or selective erosion in upper horizons;
4. what has greater effect on plant growth and thus vegetation growing on particular soils: the observed evidence of clay iluviation simultaneously with rather small clay ratio between Bt and E horizon (for example 1,2), or rather clay gradient alone (of about 1,5? I think, that the water movement in soil (infiltration) is more affected by clay gradient, that clay skins or similar features. Although, some laboratory, model works would be needed to verify this.
Consequently, as more agronomist than soil scientist, when I must identify soil on field or after textural analysis, I would prefer to classify majority of soils with textural gradient AND without evidence of lithological discontinuity as soils with argillic horizon. Anyway, according to WRB 2014: "The classification of soil is based on soil properties defined in terms of diagnostic horizons, diagnostic properties and diagnostic materials which to the greatest extent possible should be measurable and observable in the field".
Sir,
Besides Bt horizon analysis, above layer needs to be analysed for texture.
Thanks
Thanks Michal for your extensive interventions. It is a learning for me.I fully agree to your views expressed in your last sentence. Difficulty in identifying clay skins is common not only in Indian IGP soils ( few pdfs are enclosed for your perusal) but also elsewhere ( McKeague et al., 1981. Geoderma, 25: 63-74). We need to have a set of analyticaly sound criteria that will help in precise and unambiguous definitions of soil taxa.
Dilip, one of the issues here is that the diagnostic criteria were originally developed for US soils and although there has been an attempt to export it to the rest of the world (and to adapt as this occurs) - dilemmas such as yours are inevitable. You are trying to squeeze a square peg into a round hole. How you tackle this depends on the purpose of the classification in the first place (mapping, understanding soils for management, sharing information with others - as so clearly articulated by Michal). If the exact criteria for soil taxonomy doesn't work for you in a meaningful manner that helps you classify soils so that you can provide management advice based on a classification that groups soils of similar behaviour together, then it may not be fit for purpose and you would be best to come up with a definition/criteria that does work for you.
Referring to your last sentence .Very true Andrew. Thats what we are attempting.
I agree with all, the laboratory texture analysis, although cannot be made on the field, is necessary, or at least very useful, because is much more reliable, than field texture assessment. I refer to hydrometric and pipette method, but not the laser diffraction method.
Dilip, thank you very much for the papers regarding clay illuviation in Indian soils.
I think that soil classification systems are imperfect and must be/are constantly improved according to the progress of knowledge. Still we don't know all soil existing in the World, and they are changing, and the methods of study are also being improved.
In my opinion, if argic/argillic horizon may be formed not only by clay illuviation, but also by it's formaton in situ or/and by clay destruction or selective clay erosion in upper horizons, the main criterion for denomination of this horizon should be clay gradient between Bt and neighbouring upper layer (not only E, but also A), even, if clay skins or similar features are not observed. These criteria (clay skins etc) should/must be applied at lower classification levels than Soil Order or Soil Reference Group.
Clay illuviation as we all know is better quantified using laboratory/micromorphlogy data. It becomes difficult at time to identify clay skins on the field. Horizon designations can be changed as per the properties, but designations should be given based on the intrinsic properties which do not change in short interval of time.
I agree to some views which indicated a change in soil classification system as e.g., all soils are Alfisols if there is a clay illuviation of 1.2 times, etc. and a base saturation not as low as 35%, irrespective of their degree of weathering, colour, mineral suit, etc. This point need to be argued more vigorously to explain the varied land uses within the Alfisols
Dr. Ray- Thanks for your invention.At first,I would however look in to the basics of clay illuviation process(CIP) in soils of the Indo-Gangetic Plains (IGP) under semi-arid climate (SAT). This process in other soil types (Vertisols and red ferruginous soils of both SAT and humid climates) can be taken at a later stage of interaction among our distinguished colleagues. In later soils understanding CIP is a bit complicated.
I agree with contributors. clay skin (cutan) to me should not be emphasized as it is difficult to identify it on the field. our research using micromorphology, revealed that what we have most in tropical environment is stress cutan which are only visible under micro. I think we should depend on textural.
Dear Gabriel- Thank you for your comments/suggestions. Hope we will soon reach to a general agreement of not considering the clay skin criterion.
Dr. Pal thanks for the detail clarification which I read many times from your paper. Clay skin criteria should not be removed as it would otherwise become difficult for field personnel to identify the presence of argillic horizon on the field. Instead absence of clay skins should not be the only criteria to prove that there has not been sufficient clay illuviation so as to classify a particular soil to Alfisol, etc. order.
My previous comment about change in classification in various Alfisols within the IGP of SAT was based on Micheal's comments.
Dr. Ray- Our research results (please refer to the attached papers) provide a deductive check on the inductive reasoning on the . formation of argillic horizon,which hitherto based on identification of clay skins in the field alongside textural fulfillment. It is to be noted that clay enrichment has also been contributed to sedimentation processes , geogenic origins , in situ weathering of biotite particles. Therefore, relying on the presence of identifiable clay skins, an immediate decision to assign argillic horizon would still remain fallible.
Dear colleagues,
as we know, the origin and definition of Bt horizon has been continuously and intensively discussed at least since 50s of 20th century. As Andrew stated, initially the discussion based on northern (European/American) soils with little attention to southern soils of more dry/warm climates. So, the definitions refer mainly to materials, conditions and processes known from temperate climates. More recently, e.g. due to increasing activity of pedologists from Asian, African and South American countries, we revise "old" definitions. This was done in FAO - WRB 2014/2015 classification, which allowed the argic horizon recognition without clay skins/bridges occurrence; however, any lithological discontinuity must be excluded (taking into account the criteria of textural discontinuity given in WRB). This was made after numerous field demonstrations and long-term discussion, because we learned that the illuviation may not process in the same way as under temperate climate, and it may be accompanied by other processes/phenomena, and the results (clay skins) may be intensively damaged due to biological or clay activity.
But we can ask question: is this horizon an analogue of "temperate" argic/argillic? Maybe we should look for a new formula (new diagnostic horizon) instead of reinterpret the concept of argic/argillic diagnostic horizon and artificially broaden the original definition of this horizon? We invented several ("new") diagnostic horizons for specific soils of tropics, so why we shouldn't start the discussion about something new between argic/argillic and cambic? We only need clear arguments that the observed (and measured) features are pedogenic and specifically different from the features (a set of features) assigned for cambic/argic-argillic.
Dr. Kabala- Excellent. Truly we need to develop solid arguments in favour of textural fulfillment for a Bt horizon as a result of pedogenic processes. We did some research in this direction and some papers are already attached. Please refer to those and you may find them useful information base.
Argument of Dr. Kabala is interesting and clay illumination per se should not be solely considered in the tropical terms. Temperate clay illuviation (orgnans, etc in addition to argillans) promises to open new areas of research. It may also be noted that contemporary clay illuviation in the semi-arid parts of India should not be considered as a result of typical tropical clay illuviation.
@Michal
As proposed by Stolt and Baker (1994), C layer (saprolithic material, see definition in the linked paper) lacks illuvial clay observable with a hand lens in small macrovoids, howeverclay coatings in large macropores and along geogenic features such as fractures, cracks, or sedimentary beds may occur (Juilleret et al, 2016).
Stolt, H.M., Baker, J.C., 1994.Strategies for studying saprolite and saprolite genesis. In:Cremeens, D.L., Brown, R.B., Huddleston, J.H. (Eds.), Whole Regolith Pedology: SoilScience Society of America Special Publication Number 34, pp. 1
–19.
Article Mind the gap: A classification system for integrating the su...
Dear Jerome- Thanks for sharing the pdf and joining in the discussion.
Dear Ray- It is true that clay illuviation processes in SAT calcareous soils needs special attention as such processes are contemporary during the dry period of the Holocene period.
Dear Dr. Ray- The dispersion of clay particles is possible under slightly acidic to moderately alkaline pH conditions at a very low electrolyte concentration that ensures a pH higher than that required for the zero point of charge for complete dispersion of clay. Therefore, the movement of deflocculated clays and its subsequent accumulation in the Bt horizons occurs at the initial stage of soil formation when the pH is moderately alkaline and remains above the point of zero charge. Clay colloids remain dispersed in alkaline pH condition and thus clay illuviation is a contemporary process in SAT calcareous soils.But in both humid temperate and tropical climate it is no more operative in the present soil acidic conditions and therefore their clay pedofeatures
are not considered to be the result of current pedogenic processes. You may refer to the section 3. 'Clay enriched B horizons, clay illuviation and taxonomic rationale
of RF soils of HT climate' of the attached pdf.
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